Is neatly in this sentence an adverb or adjective?

In the sentence "He arranged the books neatly on the shelf," neatly is an adverb.

Here's why:

* Adverbs modify verbs, adjectives, or other adverbs. They tell us how, when, where, or to what extent something is done.

* Adjectives describe nouns or pronouns.

In this case, "neatly" describes how he arranged the books. It modifies the verb "arranged".